External assessment of myocardial metabolism with C-11 palmitate in vivo

The externally detected rate of clearance of C-11 palmitate ([11C]palmitic acid) from isolated hearts varies directly with CO2 production from neutral lipids and with physiological indexes of myocardial oxygen consumption. The present study was performed to determine whether myocardial metabolism could be quantified noninvasively in vivo in a fashion analogous to that in the isolated heart. Opened chest, male rabbits were injected with C-11 palmitate (100-200 muCi) and coincidence counts were detected externally with two NaI(TI) crystals so placed that their colinear field of view encompassed the heart. The monoexponential rate of clearance of tracer--obtained from the portion of the residue-detection curve reflecting metabolism of fatty acid incorporated into neutral lipids--correlated directly with induced changes in tension-time index after injections into the left atrium (r = 0.96, n = 12), right atrium (r = 0.86, n = 14), and ear vein (r = 0.93, n = 14). Clearance of labeled palmitate from the vascular pool within the field of detection (determined with both C-14 palmitate and red blood cells labeled with C15O-hemoglobin) was rapid and did not significantly affect measurements of palmitate clearance from the heart itself.